1. Field of the Invention
The present invention relates to a construction of a spindle motor for use in magneto-optical disc-driving devices of information processing apparatuses.
2. Description of the Prior Art
A DC brushless type motor of an ordinary outer rotor type has generally been used in the spindle motor that rotatably drives magneto-optical discs. It is conventional that the magneto-optical discs are attracted and held at their predetermined positions by means of a magnetic disc clamping device fixed around an end of the rotary shaft of the spindle motor. The magneto-optical disc then is driven or rotated by the spindle motor.
FIG. 2 shows a construction of the spindle motor for magneto-optical disc driving according to the prior art. In the drawing, the reference numeral 1 shows a magnetic shield fixed at a side of the stator of the spindle motor, which is also used as a motor fixing frame. To the boss of the shield 1, a rotary shaft 3 is journalled through a bearing 2. A cup-like rotor yoke 4 with a central saucer-shaped section is fitted to an end of the rotary shaft 3, and the rotor yoke 4 is attached to the rotary shaft 3 by an adhesive applied to a groove 5 formed around the shaft 3. Permanent field magnets 6 are secured to the inner circumferential face of the cylindrical portion of the rotor yoke 4 in such a way that the poles of N and S are alternatively arranged. A stator 7 is arranged on the circumference of the boss of the shield 1 and the outer peripheral face of the stator 7 faces the field magnets 6 leaving a small radial gap. Hall elements 8 detect a magnet positions so as to determine the rotating angle of the rotor yoke 4. The Hall elements 8 are secured on the print substrate 9 of a control circuit at the side of the shield so as to face the axial end of the magnet 6.
The conventional DC brushless motor having the aforementioned construction has been well known. In accordance with the rotating angle determined, the control circuit controls the timing of supplying currents to the stator coil according to the predetermined order, and drives the rotor in a fixed direction and at a constant speed.
In order to drive (rotate) the magneto-optical disc held on the rotary shaft of the motor, a magnetic disc clamp 10 is installed at the end of the rotary shaft 3 extending through the shield 1. The disc clamp 10 consists of a cup-shaped clamp case 11, and a ring-shaped disc attracting magnet 12, fixed within the clamp case 11 so as to attract the magneto-optical disc. (The magneto-optical disc is held at a side end of the clamp case 11). The disc clamp 10 is firmly secured by fitting the clamp case 11 onto the rotary shaft 3 and applying an adhesive to a groove 13 formed on the rotary shaft 3. The N pole and S pole of the disc attracting magnet 12 are positioned along the axial direction of the rotary shaft 3.
The mounting operation of the magneto-optical discs on the spindle motor through the disc clamp 10 will now be explained. A mounting disc 16 of a magnetic material is attached to the magneto-optical disc 14 by means of a boss 17. Each mounting disc has a central hole 15 through which the rotary shaft 3 is inserted. The magneto-optical disc 14 is accommodated in a cassette case (not shown). First, the magneto-optical disc 14 is inserted on the rotary shaft 3 as is accommodated in the cassette case, along the direction P shown by an arrow. Then, the mounting disc 16 is attracted to the disc attracting magnet 12 of the disc clamp 10 and the surface of the magneto-optical disc 14 contacts with an end face of the clamp case 11, resulting in the fixed mounting of the magneto-optical disc 14 at the predetermined position. The magneto-optical disc 14, mounted at the predetermined position on the shaft 3, is driven as the spindle motor drives the rotary shaft 3. At the demounting, the magneto-optical disc 14 can be displaced from the disc clamp 10 by applying a force to the cassette case along a direction opposite to direction P.
The specification of the disc drive spindle motor, including its dimensions and functions follows:
(1) Because of the restriction on the total height of the disc driving apparatus and the necessity to mount the magneto-optical disc on the spindle motor together with the disc cassette case, the axial length L2 as shown in FIG. 2 must be as short as possible (for example, L2 should be less than 23 mm);
(2) The spindle motor must drive a magnetooptical disc at a constant and fixed rotation speed (1800 rpm); and
(3) The tilt of the motor driven rotary shaft 3 and rolling of the disc plane must be very small when the magneto-optical disc is mounted and driven by the motor. (Permissible rolling value at the outer peripheral edge of a magneto-optical disc is less than 0.1 mm, the converted value in terms of shaft oscillation is less than 10 microns).
However, it is difficult to completely satisfy the above required specifications by the conventional construction of the spindle motor. The detailed reasons for this unsatisfactory condition will be described.
(1) Because a disc clamp 10 is mounted on the rotary shaft of the motor as a discrete or individual part, the total axial length L2 of the motor becomes too long to satisfy the specifications in the design of the motor. In order to solve the short-comings of the prior art, a small, strong magnet, having a large magnetic attractive force, can be employed for the disc attracting magnet 12, resulting in a corresponding shortened size of the disc clamp 10. However, the strong magnet is expensive, resulting in increased total cost of manufacturing the motor.
(2) In a conventional motor, the rotor yoke 4 and the disc clamp 10 are mounted on both ends of the rotary shaft separately, with bearings 2 between them as shown in FIG. 2, increasing rotary unbalance, resulting in the generation of shaking and vibration. For practical reasons, the motor products are shipped after the rotary balance of the motors has been adjusted. The separately mounted two rotating bodies, rotor yoke 4 and disc clamp 10, create difficulty in balancing them. It is nearly impossible to obtain a complete balance by the adjustment described above. Therefore, some unbalanced condition will always exist between the rotary parts of the rotor yoke 4 and the disc clamp 10. Depending on the degree of the unbalance or maladjustment, the precession is induced to cause the rotary shaft to vibrate severely.
(3) As the fit length of the rotor yoke 4 and the rotary shaft 3 is designed small in order to shorten the total axial size of the motor, the axis of the rotor yoke 4 tends to have some angle to the axis of the rotary shaft 3 for some fitting allowances when assembled. This results again in shaft vibration.
(4) Since the disc clamp 10 is an individual part separate from the motor proper, the number of parts to be assembled in manufacturing the spindle motor and the number of the motor manufacturing steps is increased, resulting in increased cost of the motor.